Recently, in view of the exhaustion of oil resources in the future, the usefulness primitive carbonaceous substances, such as coal and tar sand, which are now the most abundant fossil fuel sources and widely distributed all over the world have been reconsidered. These substances have received increasing attention as an energy source and a chemical feed capable of replacing petroleum. However, coal is a very complicated polymeric compound, and contains fairly large amounts of hetero atoms, such as oxygen, nitrogen, and sulfur, and ash, as well as carbon and hydrogen which are the major constitutive elements. Therefore, coal, when burned as such, produces large amounts of air pollution substances. Furthermore, coal is not desirable because its calorific value is low as compared with petroleum, and the transportation and storage of coal is cumbersome and expensive.
In order to overcome the above-described substantial problems of coal, a number of methods of liquifying coal have been proposed in which the coal is liquified to remove the hetero atoms and ash, and to produce fuel oils or gases causing no air pollution as well as chemical starting materials of high practical value. Typical examples include a method in which coal is extracted with a solvent (see U.S. Pat. No. 4,022,680), a method in which coal is liquified in the presence of hydrogen or a hydrogen-donating compound (see U.S. Pat. No. 4,191,629 and W. German Pat. No. 2,756,976), a method in which coal is liquified and gassified in the presence of hydrogen (see U.S. Pat Nos. 3,152,063, 3,823,084, 3,960,700, 4,169,128, 3,985,519 and 3,923,635), and a method in which coal is liquified and gassified in an inert gas (see U.S. Pat. No. 3,736,233).
In accordance with these methods, however, it is not possible to mainly and efficiently produce a gasoline fraction which is to be used as a fuel for transportation and a chemical feedstock, although the methods can directly produce those ingredients which can be used as an energy source.
A method of directly producing a gasoline fraction which has been known involves injecting finely ground coal in a high temperature and pressure hydrogen stream to achieve high-speed hydrogenation and thermal cracking of the coal in a short period of time of from several ten milliseconds to several minutes. More specifically the finely ground coal is injected into a hydrogen stream having a pressure of from 50 to 250 kg/cm.sup.2 (gauge pressure) and a temperature of from 600.degree. to 1,200.degree. C. The coal is heated rapidly at a rate of from 10.sup.2 to 10.sup.3 .degree. C./sec to achieve the hydrogenation and thermal cracking. This method produces gas products such as methane, ethane, carbon monooxide, carbon dioxide, steam, hydrogen sulfide and ammonia, liquid products such as a gasoline fraction and heavy oil (comprising aromatic compounds containing at least 10 carbon atoms and high boiling tar), and a solid product containing ash, which is called "char".
This method, however, has various disadvantages. For example, when the reaction temperature is lowered, the total conversion of coal into liquid or gas is decreased, the total conversion being given by the following formula: ##EQU1## Furthermore, heavy oils such as aromatic compounds containing at least 10 carbon atoms and tar are produced as main products. On the other hand, when the reaction temperature is raised, although the total conversion is increased, decomposition of liquid products is accelerated, resulting in the production of methane as a main product. This leads to a reduction in the gasoline fraction conversion; i.e., the conversion is at most from 3 to 8%.
As a result of extensive studies on the production of the gasoline fraction to be used as a fuel for transportation or a chemical feedstock from carbonaceous substances, such as coal, in high yield, it has been found that:
(1) the gasoline fraction is produced not only directly from the carbonaceous substances, but also from liquid products, intermediate products produced in the course of thermal cracking, which are further hydrocracked and converted into lighter products;
(2) the proportion of the gasoline fraction produced from the liquid products in the total gasoline fraction is much greater than that of the gasoline fraction produced directly from the carbonaceous substances;
(3) therefore, in order to increase the conversion of the carbonaceous substances into the gasoline fraction, it is necessary to increase the amount of the liquid products, i.e., gasoline fraction precursors, being formed; and
(4) when the carbonaceous substances are thermally cracked in the presence of certain kinds of metal compounds, the thermal cracking is accelerated, and the conversion of the carbonaceous substances into the liquid products is increased.
Furthermore, it has been found that not only the conversion of the carbonaceous substances into the gasoline fraction is increased, but also the conversion of the carbonaceous substances into light oils is greatly increased. The term "light oil" as used herein refers to an oil composed mainly of from 2 to 5 ring-condensed aromatic compounds.